Pneumatic tire

ABSTRACT

The present invention has an object of providing a pneumatic tire having a tread pattern capable of providing both excellent drivability and a high partial wear resistance. The pneumatic tire of the present invention includes, in a tread surface, a compound groove partitioned into a first groove portion and a second groove portion by a first groove side and a second groove side, the first groove side having an oblique side portion diagonally disposed relative to a tire circumferential line in plan view, the second groove side intersecting with the oblique side portion of the first groove side at an acute angle in plan view, in which: the first groove portion and the second groove portion both have a groove depth that gradually reduces in an extending direction of the groove toward an intersection between the oblique side portion and the second groove side; and a groove wall extending from the first groove side is formed as a tapered surface mildly-sloped toward a groove bottom on the intersection side when viewed in the groove transverse direction. Further, the pneumatic tire of the present invention includes, in a tread surface, a groove partitioned by at least three groove sides in plan view, in which: one or more of groove walls extending from one of the at least three groove sides that extends in the longitudinal direction of the groove is formed as a tapered surface at least in part thereof; and the tapered surface forms an angle with a plane perpendicular to the tread surface, which varies in the extending direction of the groove side.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2011/003505, filed on Jun. 20, 2011, which claims priority fromJapanese Patent Application No. 2010-152421, filed on Jul. 2, 2010, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a pneumatic tire, and more particularlyto a pneumatic tire suitably used as a pneumatic tire for a motorcycle.

BACKGROUND ART

In general, a pneumatic tire, in particular, a pneumatic tire for amotorcycle is known to be largely affected, in terms of drivability, bythe shape of grooves (tread pattern) formed in a tread surface of atread portion.

As an example of a conventional pneumatic tire for a motorcycle, therehas been proposed a pneumatic tire for a motorcycle having acircumferential groove extending substantially in the tirecircumferential direction and a plurality of lateral grooves extendingsubstantially in the tire width direction, the grooves being formed in atread surface of the tire (see, for example, Patent Literatures 1, 2).In the aforementioned pneumatic tire for a motorcycle, thecircumferential groove functions to improve drivability and to preventskidding while the lateral grooves exert drive force and braking force.

However, the grooves formed in a tread surface of the tread portion,which may improve the drivability or the like, also reduces the rigidityof the tread portion to generate a difference in rigidity. As a result,a pneumatic tire for a motorcycle having a circumferential groove and alateral groove formed in a tread surface of a tread portion for thepurpose of ensuring drivability can be subjected to significant partialwear, and thus it has been difficult to ensure both drivability andpartial wear resistance at the same time.

CITATION LIST Patent Literature

-   PTL 1: JP H04-321404 A-   PTL 2: JP H06-115316 A

SUMMARY OF INVENTION Technical Problem

In view of the above, there has been a demand for a pneumatic tirehaving a tread pattern capable of ensuring both excellent drivabilityand high partial wear resistance at the same time.

Solution to Problem

The present invention has an object to advantageously solve theaforementioned problems, and a pneumatic tire of the present inventionhas a feature of including, in a tread surface, a compound groovepartitioned into a first groove portion and a second groove portion by afirst groove side and a second groove side, the first groove side havingan oblique side portion diagonally disposed relative to a tirecircumferential line in plan view, the second groove side intersectingwith the oblique side portion of the first groove side at an acute anglein plan view, in which the first groove portion and the second grooveportion both have a groove depth that gradually reduces in an extendingdirection of the groove toward an intersection between the oblique sideportion and the second groove side, and a groove wall extending from thefirst groove side is formed as a tapered surface mildly-sloped toward agroove bottom on the intersection side when viewed in a groovetransverse direction. As described above, the groove depth is madesmaller on the intersection side in the groove extending direction whilea tapered surface is formed on the intersection side in the groovetransverse direction, to thereby suppress generation of partial wear andreduction in rigidity, particularly in the vicinity of the oblique sideportion. Therefore, a high partial wear resistance can be obtained whileproviding excellent drivability.

Here, the “compound groove partitioned into a first groove portion and asecond groove portion by a first groove side and a second groove side”in the present invention may include, without impairing the effect ofthe invention, a compound groove partitioned into the first grooveportion and the second groove portion that are slightly spaced apartfrom each other due to manufacturing reasons. That is, the first grooveside and the second groove side may not be technically connected at theintersection, whereas a virtual line connecting the disconnected firstgroove sides and a virtual line connecting the disconnected secondgroove sides still intersect with each other to form the intersection.Further, the “groove extending direction” refers to an extendingdirection, in plan view, of a line passing through the center of thecompound groove width measured in a direction perpendicular to theoblique side portion and the extension thereof, and the “groovetransverse direction” refers to a direction, in plan view, perpendicularto the oblique side portion and the extension thereof.

Here, in the pneumatic tire of the present invention, the oblique sideportion is preferably shorter in length than the second groove side. Theoblique side portion shorter in length than the second groove sideallows the tapered surface to extend across the most part of the firstgroove portion and second groove portion, which further improves thepneumatic tire in partial wear resistance.

Further, in the pneumatic tire of the present invention, the taperedsurface forms an angle with a plane perpendicular to the tread surface,the angle preferably falling within a range of 20° to 75° measured fromthe acute-angle side. The tapered surface needs to form an angle of atleast 20° with a plane perpendicular to the tread surface in order tosufficiently suppressing reduction in rigidity in the vicinity of theoblique side portion to thereby improve partial wear resistance. On theother hand, the angle needs to be 75° at maximum in order to ensuredrainage, and also to avoid unnecessarily increasing the groove arearequired for forming the tapered surface, to thereby suppress reductionin rigidity. It should be noted that, in the present invention, “thetapered surface forms an angle with a plane perpendicular to the treadsurface”, which means that the tapered surface forms an angle with avirtual plane passing through the oblique side portion in a directionperpendicular to the tread surface.

In addition, in the pneumatic tire of the present invention, the obliqueside portion forms an inclination angle relative to the tirecircumferential line, the inclination angle preferably falling within arange of 15° to 85° measured from the acute-angle side. The inclinationangle less than 15° may fail to ensure sufficient rigidity in the tirecircumferential direction in the vicinity of the oblique side portion.On the other hand, the inclination angle over 85° may fail to ensuresufficient rigidity in the tire width direction in the vicinity of theoblique side portion.

Then, in the pneumatic tire of the present invention, the first grooveportion and the second groove portion both preferably have a groove wallextending from the second groove side, the groove wall forming an anglewith a plane perpendicular to the tread surface, which preferably variesin an extending direction of the second groove side. The angle of agroove wall extending from the second groove side may be adapted tochange, so as to obtain a high partial wear resistance and excellentdrivability, while providing excellent turning performance. It should benoted that, in the present invention, the “groove wall extending fromthe second groove side forms an angle with a plane perpendicular to thetread surface”, which means that the groove wall extending from thesecond groove side forms an angle with a virtual plane passing throughthe second groove side in a direction perpendicular to the treadsurface. Further, the “virtual plane passing through the second grooveside” refers to a virtual plane passing through a tangent of the secondgroove side when the second groove side is a curved line.

The present invention has an object to advantageously solve theaforementioned problems, and a pneumatic tire of the present inventionhas a feature of including, in a tread surface, a groove partitioned byat least three groove sides in plan view, in which the groove sidesextending in a longitudinal direction of the groove have groove wallsextending toward a groove bottom, one or more of the groove walls beingformed as tapered surfaces at least in part thereof; and in which thetapered surfaces each form an angle with a plane perpendicular to thetread surface, which varies in an extending direction of the grooveside.

It should be noted that, in the present invention, “the tapered surfaceforms an angle with a plane perpendicular to the tread surface”, whichmeans that the tapered surface forms an angle with a virtual plane whichis perpendicular to the tread portion as passing through a groove sideextending in a longitudinal direction of the groove. Further, the“virtual plane passing through a groove side” refers to a virtual planepassing through a tangent of the groove side when the groove side is acurved line. In addition, the “angle which varies in an extendingdirection of the groove side” may refer not only to an angle whichvaries continuously but also to an angle which varies intermittently.

Then, the pneumatic tire of the present invention preferably includes atleast one groove wall formed of a mildly-sloped surface that intersectswith the tapered surface within the groove.

Advantageous Effect of Invention

According to the present invention, there can be provided a pneumatictire capable of ensuring both excellent drivability and a high partialwear resistance.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be further described below with reference tothe accompanying drawings, wherein:

FIG. 1 is a development view illustrating part of a tread portion of arepresentative example of a pneumatic tire according to the presentinvention;

FIG. 2 is an enlarged view illustrating in magnification one compoundgroove formed in a tread portion of the pneumatic tire of FIG. 1;

FIGS. 3( a) to 3(e) are sectional views each taken along the lines A-Ato E-E, respectively, of FIG. 2; and

FIG. 4 are explanatory views each for illustrating forces to be appliedto the pneumatic tire of FIG. 1 when rolled under load, in which: FIG.4( a) illustrates the pneumatic tire of FIG. 1 mounted onto a frontwheel of a motorcycle; and FIG. 4( b) illustrates the pneumatic tire ofFIG. 1 mounted onto a rear wheel of a motorcycle.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Here, FIG. 1 is a developmentview illustrating part of a tread portion of an example of a pneumatictire according to the present invention. The pneumatic tire of FIG. 1,which illustrates the tread portion thereof in part, is a pneumatic tirefor a motorcycle, and more specifically, a pneumatic tire for amotorcycle designed for racing. The pneumatic tire for a motorcycle hasa plurality of compound grooves 2 formed in a tread surface 1 thereof,the grooves 2 each terminating within the tread surface 1. Specifically,this exemplary pneumatic tire has a plurality of compound grooves 2 inthe same shape formed in the tread surface 1 thereof, the compoundgrooves 2 being dimensionally arranged as being oriented opposite toeach other (line symmetry) across a tire equator E while being displacedby half a pitch from each other in the tire circumferential direction.

Here, as illustrated in FIGS. 1 and 2, the compound grooves 2 are eachpartitioned into a first groove portion 5 and a second groove portion 6by a first groove side 3 and a second groove side 4, the first grooveportion 5 and the second groove portion 6 being substantially triangularin shape. In other words, the compound grooves 2 each include a pair ofgroove portions (the first groove portion 5 and the second grooveportion 6).

The first groove side 3 bends twice to extend substantially in the tirecircumference direction. Specifically, the first groove side 3 includes:a first side portion 31 extending from one end (lower side of FIGS. 1and 2) in the tire circumference direction of the compound groove 2; asecond side portion 32 extending from the other end (upper side in FIGS.1 and 2) in the tire circumferential direction; and an oblique sideportion 33 connecting the first side portion 31 and the second sideportion 32 on the center side of the compound groove 2 in the tirecircumferential direction while being obliquely disposed at aninclination angle of a relative to a tire circumferential line parallelto the tire equator E.

Further, the second groove side 4 is a curved line extendingsubstantially in the tire circumferential direction as being convexed tothe tire equator E side. Then, the second groove side 4 intersects, atan intersection C, with the oblique side portion 33 of the first grooveside 3 at an angle β or (0°<β<90°). Here, in this exemplary pneumatictire, the second groove side 4 is a curved line, and therefore, theintersection angle β between the oblique side portion 33 and the secondgroove side 4 refers to an angle between the oblique side portion 33 anda tangent of the second groove side 4 at the intersection C.

The first groove portion 5 is, as illustrated in FIGS. 1 and 2, aportion which is substantially triangular in shape in plan view enclosedby the first side portion 31, part of the oblique side portion 33, andpart of the second groove side 4.

Then, in the first groove portion 5, the groove walls are each formed inpart as a mildly-sloped tapered surface. Specifically, in the firstgroove portion 5, part of a groove wall extending from the first sideportion 31 of the first groove side 3 toward the groove bottom, a groovewall extending from the oblique side portion 33 toward the groovebottom, and part of a groove wall extending from the second groove side4 toward the groove bottom each are configured as mildly-sloped taperedsurfaces T1 to T3, respectively, each being in a shape indicated withhatched lines in FIG. 2 in plan view.

Specifically, in the first groove portion 5, as illustrated in sectionsin FIGS. 3( a) and 3(b) which are taken along the lines A-A and B-B ofFIG. 2, respectively, a groove wall extending from the oblique sideportion 33 on the intersection C side of the first groove side 3 whenviewed in a groove transverse direction (direction perpendicular to theoblique side portion 33 and to an extension thereof) toward a groovebottom is formed as the mildly-sloped tapered surface T1 which forms anangle γ1 (on the acute angle side) with a plane passing through theoblique side portion 33 in a direction perpendicular to the treadsurface 1. Further, in the first groove portion 5, as illustrated insections in FIGS. 3( c) and 3(d) which are taken along the lines C-C andD-D of FIG. 2, respectively, part of a groove wall extending from thefirst side portion 31 of the first groove side 3 toward the groovebottom is formed as the mildly-sloped tapered surface T2, and part of agroove wall extending from the second groove side 4 toward the groovebottom is formed as the mildly-sloped tapered surface T3. Therefore, thefirst groove portion 5 has a groove depth, as illustrated in FIGS. 3( a)to 3(c), which gradually decreases in a groove extending direction(extending direction of a line passing through the center of a groovewidth measured in a direction perpendicular to the oblique side portion33 and the extension thereof) toward the intersection C.

Here, as is apparent from FIGS. 2 and 3( a) to 3(c), the tapered surfaceT3, which is part of the groove wall extending from the second grooveside 4 of the first groove portion 5 toward the groove bottom, forms anangle with a plane perpendicular to the tread surface, which varies inthe extending direction of the second groove side 4. Specifically, whenviewed in section in a direction perpendicular to the extendingdirection of the second groove side 4, the angle formed between thetapered surface T3 and the tread surface becomes larger on theintersection C side. Further, the tapered surface T2, which is part of agroove wall extending from the first side portion 31 of the first grooveside 3 toward the groove bottom, also forms an angle with a planeperpendicular to the tread surface which changes in the extendingdirection of the first side portion 31. Specifically, when viewed insection in a direction perpendicular to the extending direction of thefirst side portion 31, the angle formed between the tapered surface T2and the tread surface becomes larger on the oblique side portion 33side.

The second groove portion 6 is, as illustrated in FIGS. 1 and 2, aportion which is substantially triangular in shape in plan view enclosedby the second side portion 32, part of the oblique side portion 33, andpart of the second groove side 4.

Then, in the second groove portion 6, as in the first groove portion 5,the groove walls are formed in part as mildly-sloped tapered surfaces.Specifically, in the second groove portion 6, part of a groove wallextending from the second side portion 32 of the first groove side 3toward the groove bottom, a groove wall extending from the oblique sideportion 33 toward the groove bottom, and part of a groove wall extendingfrom the second groove side 4 toward the groove bottom each areconfigured as mildly-sloped tapered surfaces T4 to T6, respectively,each being in a shape as indicated with hatched lines in FIG. 2 in planview.

Specifically, in the second groove portion 6, as illustrated in sectionsin FIG. 3( e) which is taken along the line E-E of FIG. 2, a groove wallextending from the oblique side portion 33 on the intersection C side ofthe first groove side 3 when viewed in a groove transverse direction(direction perpendicular to the oblique side portion 33 and to anextension thereof) toward a groove bottom is formed as the mildly-slopedtapered surface T4 which forms an angle γ2 (on the acute angle side)with a plane passing through the oblique side portion 33 in a directionperpendicular to the tread surface 1. Further, in the second grooveportion 6, as in the first groove portion 5, part of a groove wallextending from the second side portion 32 of the first groove side 3toward the groove bottom is formed as the mildly-sloped tapered surfaceT5, and a groove wall extending from the second groove side 4 toward thegroove bottom is formed in part as the mildly-sloped tapered surface T6.Therefore, the second groove portion 6 has, similarly to the firstgroove portion 5, a groove depth which gradually decreases toward theintersection C when viewed in a groove extending direction (extendingdirection of a line passing through the center of a groove widthmeasured in a direction perpendicular to the oblique side portion 33 andthe extension thereof).

Here, the tapered surface T6, which is part of the groove wall extendingfrom the second groove side 4 of the second groove portion 6 toward thegroove bottom, forms an angle with a plane perpendicular to the treadsurface, which varies in the extending direction of the second grooveside 4. Specifically, when viewed in section in a directionperpendicular to the extending direction of the second groove side 4,the angle formed between the tapered surface T6 and the tread surfacebecomes larger on the intersection C side. Further, the tapered surfaceT5, which is part of a groove wall extending from the second sideportion 32 of the first groove side 3 toward the groove bottom, alsoforms an angle with a plane perpendicular to the tread surface whichchanges in the extending direction of the second side portion 32.Specifically, when viewed in section in a direction perpendicular to theextending direction of the second side portion 32, the angle formedbetween the tapered surface T5 and the tread surface becomes larger onthe oblique side portion 33 side.

Meanwhile, a motorcycle generally has a feature in that it makes a turnby banking the body thereof. Accordingly, a tire mounted on a motorcyclecomes into contact in different parts thereof with a road surface,depending on the bank (camber angle) of the body. Further, the tiremounted on a motorcycle is subject to a large lateral force such ascamber thrust when the body is banked, in addition to forces to beapplied in an anteroposterior direction (tire circumferential direction)such as a braking force and a driving force.

Therefore, when the pneumatic tire of the present invention is appliedto a front wheel and a rear wheel of a motorcycle, for example, thefront wheel is subject to forces in directions indicated by the arrowsof FIG. 4( a) while the rear wheel is subject to forces in directionsindicated by the arrows of FIG. 4( b). Here, the forces are input indifferent directions between the front wheel and the rear wheel of amotorcycle, and thus, in applying the pneumatic tire of the presentinvention, the tires are mounted in opposite directions between thefront wheel and the rear wheel, as illustrated in FIGS. 4( a) and 4(b).

Then, as described above, the first groove portion 5 and the secondgroove portion 6 of the pneumatic tire each have a groove depthgradually decreases in the groove extending direction toward theintersection C. Further, the groove wall extending from the first grooveside 3 is formed as the tapered surfaces T1, T4 that are mildly-slopedtoward the groove bottom on the intersection side C when viewed in thegroove transverse direction. Therefore, when the tire mounted onto thefront wheel is subjected forces input in directions of the arrows ofFIG. 4( a), the tapered surface T4 on the second groove portion 6 sidereceives the forces by the inclined tapered surface, to thereby suppressgeneration of partial wear. Further, the tapered surface T1 lying on thefirst groove portion 5 side supports the tread portion so as to preventthe tread portion from yielding, to thereby improve drivability. Inaddition, when the tire mounted onto the rear wheel is subjected forcesinput in directions of the arrows of FIG. 4( b), the tapered surface T4on the second groove portion 6 side receives the forces by the inclinedtapered surface, similarly to the tire mounted on the front wheel, tothereby suppress generation of partial wear. Further, the taperedsurface T1 lying on the first groove portion 5 side supports the treadportion so as to prevent the tread portion from yielding, to therebyimprove drivability. Here, in the pneumatic tire of this example, thegroove has a depth that gradually reduces in the groove extendingdirection toward the intersection C, and hence the rigidity in thevicinity of the intersection C is unlikely to be reduced despite thatthe first groove portion 5 and the second groove portion 6 are arrangedclose to each other.

Meanwhile, in the pneumatic tire of this example, the tapered surfacesT1 to T6 extending toward the groove bottom form three-dimensionalspatial structures inside the grooves, which also provides excellentappearance.

Here, in the aforementioned example of the pneumatic tire of the presentinvention, the second groove side 4 is longer in length than the obliqueside portion 33, and thus the tapered surfaces T1, T4 extend lengthwisein the groove extending direction. Accordingly, the angles γ1, γ2 formedby each of the tapered surfaces T1, T4, respectively, relative to thetread surface 1 can be ensured large. Here, the angles γ1, γ2 each maypreferably be at least 20° in view of suppressing reduction in rigidityin the vicinity of the oblique side portion 33 sufficient enough toimprove partial wear resistance. Further, the angles γ1, γ2 each may bepreferably 75° or smaller in view of obtaining a predetermined groovedepth to ensure drainage while securing a groove area necessary toarrange a mildly-sloped tapered surface with suppressing a reduction inthe rigidity. The angles γ1 and γ2 may be either the same or differentfrom each other.

Further, in the aforementioned example of the pneumatic tire of thepresent invention, in the first groove portion 5 and in the secondgroove portion 6, the groove wall extending from the second groove side4 forms a larger angle with a plane perpendicular to the tread surfaceon the intersection C side. Therefore, high partial wear resistance andexcellent drivability can be obtained while ensuring excellent turningperformance in making a turn at a particularly small camber angle.Further, the rigidity distribution of the tread portion can be optimizedto obtain excellent traction performance and braking performance.

In the pneumatic tire of the present invention, the oblique side portion33 may preferably be disposed at an inclination angle α of at least 15°relative to the tire circumferential line in view of ensuring sufficientrigidity in the tire circumferential direction in the vicinity of theoblique line portion 33. Meanwhile, the inclination angle α maypreferably be 85° at maximum in view of ensuring sufficient rigidity inthe tire width direction in the vicinity of the oblique side portion 33.

Further, in the pneumatic tire of the present invention, theintersection angle β between the oblique side portion 33 and the secondgroove side 4 may preferably be at least 5° in view of providing taperedsurfaces with sufficient area inside the first groove portion 5 and thesecond groove portion 6. On the other hand, the intersection angle β maypreferably be 45° at maximum, in view of suppressing the arrangementarea of the compound grooves from being increased excessively, whichotherwise deteriorates the rigidity.

The pneumatic tire of the present invention is not limited to theaforementioned example, but can be subjected to modifications asappropriate. Specifically, in the pneumatic tire of the presentinvention, the inclination angle α can be varied to change the extendingdirection of the compound groove 2, and alternatively, the first grooveportion 5 and the second groove portion 6 each may be configured in anarbitrary shape in plan view, and the tapered surfaces T1, T4 each maybe configured in an arbitrary shape in plan view. In addition, in thepneumatic tire of the present invention, the compound grooves 2 may beformed only in part of the tread surface 1. Alternatively, the compoundgrooves 2 may be formed in combination with other well-known treadpatterns. Still alternatively, in the aforementioned pneumatic tire, thetapered surfaces T2, T3, T5, and T6 may be dispensable. Then, thepneumatic tire of the present invention can be mounted to only one ofthe front wheel and the rear wheel of an automobile.

In the aforementioned pneumatic tire, a compound groove including afirst groove portion and a second groove portion each being in apredetermined shape is formed in a tread surface to thereby provide bothexcellent drivability and high wear resistance. However, the pneumatictire of the present invention may also be improved traction performanceand braking performance in the following manner.

Specifically, the present invention may provide a pneumatic tire havinga groove formed in the tread surface, the groove being defined by atleast three groove sides in plan view, in which one or more of groovewalls extending toward the groove bottom from a groove side extending inthe longitudinal direction of the groove is formed, at least in partthereof, as a tapered surface that forms an angle with a planeperpendicular to the tread surface, the angle changing in the extendingdirection of the groove sides.

Here, a pneumatic tire with the aforementioned configuration is notspecifically limited, and examples thereof may include a pneumatic tirehaving at least one of the first groove portion 5 and the second grooveportion 6 of the pneumatic tire of FIG. 1

Specifically, an example of the pneumatic tire with the aforementionedconfiguration may include a tire having a groove (closed groove)enclosed by a first groove side (corresponding to the first side portion31 of FIG. 1) extending substantially in the tire circumferentialdirection, a second groove side (corresponding to a part of the obliqueside portion 33 of FIG. 1, the part lying on the first side portion 31side relative to the intersection C) obliquely disposed at aninclination angle α relative to the tire circumferential line parallelto the tire equator E, and a third groove side (corresponding to a partof the second groove side 4 of FIG. 1, the part lying on the first sideportion 31 side relative to the intersection C) formed of a curved lineextending substantially in the tire circumferential direction whilebeing convexed to the tire equator E side. That is, an example of thepneumatic tire with the aforementioned configuration may include a tirewhich has only the first groove portion 5 of the pneumatic tire of FIG.1 formed therein (in other words, a tire without having the secondgroove portion 6 formed therein).

The groove of this exemplary pneumatic tire is formed, as describedabove, as being defined by the first groove side, the second grooveside, and the third groove side which are extending in differentdirections from one another, and the groove thus formed is substantiallytriangular in shape in plan view. Then, the first groove side and thethird groove side of the groove both are longer than the second grooveside.

Further, the third groove side intersects with the second groove side atan angle β or (0°<β<90°) at an intersection with the second groove side.Here, in this exemplary pneumatic tire, the third groove side is acurved line, and therefore, the intersection angle β between the secondgroove side and the third groove side refers to an angle between thesecond groove side and a tangent of the third groove side at theintersection.

Then, in the groove of the exemplary pneumatic tire, the groove wallsare each formed in part as a tapered surface. Specifically, part of agroove wall extending toward the groove bottom from the first grooveside extending in the longitudinal direction of the groove (groove walllying on the part of the second groove side), a groove wall extendingfrom the second groove side toward the groove bottom, and part of agroove wall extending toward the groove bottom from the third grooveside extending in the longitudinal direction of the groove (groove walllying on the part of the second groove side) are each configured,similarly to the groove walls of the first groove portion of thepneumatic tire of FIGS. 1 to 3, as a tapered surface.

That is, in this groove, the groove wall extending from the secondgroove side toward the groove bottom is formed as a mildly-slopedtapered surface (gentle slope) which forms, along the extendingdirection of the second groove side, a constant angle relative to aplane passing through the second groove side in a directionperpendicular to the tread surface 1.

Further, the groove wall extending from the first groove side toward thegroove bottom is formed in part as a tapered surface that forms an anglewith a plane perpendicular to the tread surface, the angle changing inthe extending direction of the first groove side. In addition, thegroove wall extending from the third groove side toward the groovebottom is formed in part as a tapered surface that forms an angle with aplane perpendicular to the tread surface, the angle changing in theextending direction of the third groove side.

Specifically, when viewed in section in a direction perpendicular to theextending direction of each groove side, the tapered surface as part ofthe groove wall extending from the first groove side toward the groovebottom and the tapered surface as part of the groove wall extending fromthe third groove side toward the groove bottom each form an angle withthe tread surface, the angle being larger on the intersection side withthe second groove side.

In addition, the tapered surface as part of the groove wall extendingfrom the first groove side toward the groove bottom and the taperedsurface as part of the groove wall extending from the third groove sidetoward the groove bottom each intersect with a mildly-sloped taperedsurface (gentle slope) serving as a groove wall extending from thesecond groove side toward the groove bottom. In other words, the taperedsurfaces form a three-dimensional spatial structure in the groove.

Therefore, this groove has a groove depth which gradually decreases in agroove extending direction (extending direction of a line passingthrough the center of a groove width measured in a directionperpendicular to the second groove side and the extension thereof)toward the intersection between the second groove side and the thirdgroove side.

Then, the pneumatic tire provided with the groove configured asdescribed above is capable of optimizing the rigidity distribution ofthe tread portion, to thereby provide excellent traction performance andbraking performance particularly in recovery to upright position fromcornering. The tire is also capable of ensuring excellent turningperformance in making a turn at a small camber angle.

Specifically, in this pneumatic tire, the groove wall on the narrowerwidth side of the groove (opposite to the side of the second groove sidein the longitudinal direction of the groove) where land portion rigidityis easy to ensure forms a small angle with the tread surface (thegradient of the groove wall is made steep) in order to ensure the groovevolume. On the other hand, the groove wall on the wider width side ofthe groove (on the part of the second groove side) where land portionrigidity is difficult to ensure forms a large angle with the treadsurface (the gradient of the groove wall is made gentle) in order toensure the land portion rigidity, because even a smaller groove depthcan ensure the groove volume. Therefore, the pneumatic tire of thisexample is capable of optimizing (uniformizing) the land portionrigidity of the tread portion, to thereby provide excellent tractionperformance, braking performance and turning performance, as compared tothe case where the inclination angle of the groove wall extending fromthe first groove side and the inclination angle of the groove wallextending from the third groove side are constant in extending directionof the groove side. Further, drivability and wear resistance can also beimproved. In addition, this pneumatic tire is capable of securing thegroove volume as compared to the case where the inclination angle of thegroove wall extending from the first groove side and the inclinationangle of the groove wall extending from the third groove side are bothconstant in the extending direction of the groove sides, to therebysuppress deterioration in drainage.

The pneumatic tire of the present invention is not limited to theaforementioned example, but can be subjected to modifications asappropriate. Specifically, the pneumatic tire of the present inventionmay include only the tapered surface extending from the third grooveside. As other examples of the pneumatic tire of the present invention,for example, the pneumatic tire of FIGS. 1 to 3 may be modified toinclude only the tapered surfaces T3 and T6 formed in the compoundgroove or to include only the tapered surfaces T2 and T5 formed in thecompound groove. In addition, in the pneumatic tire of the presentinvention, an angle (inclination angle of the groove wall) formedbetween the groove wall extending from the second groove side and aplane perpendicular to the tread surface may be varied in the extendingdirection of the second groove side. However, in view of ensuringexcellent traction performance, braking performance, and turningperformance while reducing workload required for forming the groove, theangle between the groove wall extending from the second groove side anda plane perpendicular to the tread surface may preferably be the same inthe extending direction of the second groove side.

EXAMPLES

In the following, the present invention is described in further detailwith reference to Examples. However, the present invention is no waylimited to the following Examples.

Example 1

Pneumatic tires (for rear wheels) which were configured as illustratedin FIGS. 1 to 4 and had the specifications shown in Table 1 weremanufactured as samples by a conventional method. The tires were each insize 180/55ZR17.

Then, the tires were subjected to performance evaluation according tothe following methods. The evaluation results are shown in Table 1.

Comparative Example 1

A pneumatic tire having a configuration similar to that of Example 1,except in that the angle formed between the plane perpendicular to thetread surface and the groove wall was 0° and the tapered surfaces T1 toT6 were not provided, was manufactured as a sample and subjected toperformance evaluation similarly to Example 1. The evaluation resultsare shown in Table 1.

Examples 2 to 5

Pneumatic tires each having a configuration similar to that of Example1, except in that the specifications were changed as shown in Table 1,were manufactured as samples, which were subjected to performanceevaluation similarly to Example 1. The evaluation results are shown inTable 1.

Drivability

The pneumatic tires thus manufactured were each mounted to a rim in sizeof MT5.50×17, which is assembled as a rear tire of a real vehicle of 600cc displacement at a filling air pressure of 190 kPa. The vehicle wasthen repeatedly driven at 40 km/h to 250 km/h at a camber angle of 0° to50° on a round track of 10 km, so as to evaluate drivability.Specifically, drivability was evaluated based on the feeling of aprofessional rider, which was indexed with a score of 100 representingthe drivability of Comparative Example 1. A larger index value indicatesthat the tire is more excellent in drivability.

Partial Wear Resistance

The pneumatic tires thus manufactured were each mounted to a rim in sizeof MT5.50×17, which is assembled as a rear tire of a real vehicle of 600cc displacement at a filling air pressure of 190 kPa. The vehicle wasthen repeatedly driven on a high-speed circuit at 40 km/h to 250 km/hfor 10 km at a camber angle of 0° to 50°, so as to evaluate partial wearresistance based on the wear status of the grooves. Specifically, thestate of the grooves was visually evaluated, which was indexed with ascore of 100 representing the partial wear resistance of ComparativeExample 1. A larger index value indicates that the tire is moreexcellent in partial wear resistance.

TABLE 1 Comparative Example 1 Example 1 Example 2 Example 3 Example 4Example 5 Length of Oblique Side 54 54 54 54 54 54 Portion [mm] lengthof Second Groove 250 250 250 250 250 250 Side [mm] Inclination Angle α[°] 54 54 54 54 54 54 Intersection Angle β [°] 35 35 35 35 35 35 GrooveWall Angle γl [°] 0 64 20 75 10 80 Groove Wall Angle γ2 [°] 0 72 20 7510 80 Drivability [-] 100 105 101 103 100 102 Partial Wear Resistance[-] 100 115 105 116 102 117

It can be appreciated from Table 1 that the tires of Examples 1 to 5have more excellent drivability and high partial wear resistance, ascompared to the tire of Comparative Example 1.

INDUSTRIAL APPLICABILITY

The present invention can provide a pneumatic tire having both excellentdrivability and a high partial wear resistance.

REFERENCE SIGNS LIST

-   -   1 tread surface    -   2 compound groove    -   3 first groove side    -   4 second groove side    -   5 first groove portion    -   6 second groove portion    -   31 first side portion    -   32 second side portion    -   33 oblique side portion

The invention claimed is:
 1. A pneumatic tire comprising, in a treadsurface, a compound groove partitioned into a first groove portion and asecond groove portion by a first groove side and a second groove side,the first groove side having an oblique side portion diagonally disposedrelative to a tire circumferential line in plan view, the second grooveside intersecting with the oblique side portion of the first groove sideat an acute angle in plan view, wherein the first groove portion and thesecond groove portion both have a groove depth that gradually reduces inan extending direction of the groove toward an intersection between theoblique side portion and the second groove side, and a groove wallextending from the first groove side is formed as a tapered surfacemildly-sloped toward a groove bottom on the intersection side whenviewed in a groove transverse direction.
 2. The pneumatic tire accordingto claim 1, wherein the oblique side portion is shorter in length thanthe second groove side.
 3. The pneumatic tire according to claim 1,wherein the tapered surface form an angle with a plane perpendicular tothe tread surface, the angle falling within a range of 20° to 75°measured from the acute-angle side.
 4. The pneumatic tire accordingclaim 1, wherein the oblique side portion forms an inclination anglerelative to the tire circumferential line, the inclination angle fallingwithin a range of 15° to 85° measured from the acute-angle side.
 5. Thepneumatic tire according to claim 1, wherein the first groove portionand the second groove portion both have a groove wall extending from thesecond groove side, the groove wall forming an angle with a planeperpendicular to the tread surface, which varies in an extendingdirection of the second groove side.
 6. The pneumatic tire according toclaim 1, wherein a groove wall extending from the second groove side isformed as a tapered surface mildly-sloped toward the groove bottom onthe intersection side when viewed in a groove transverse direction, andthe groove at the intersection side is formed only by the taperedsurfaces of the groove sides.